Ventricular event filtering for an implantable medical device

ABSTRACT

Pacing parameters are provided to address cross talk and intrinsic ventricular events occurring within a predefined blanking period following an atrial event. The parameters are used in conjunction with protocol for minimizing or reducing ventricular pacing, wherein ignoring intrinsic ventricular events during the blanking period might otherwise affect the performance of the protocol.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a continuation-in-part of Ser. No.10/246,816, filed Sep. 17, 2002 now U.S. Pat. No. 7,130,683, which is acontinuation-in-part of non-provisional U.S. patent application Ser. No.09/746,571 filed Dec. 21, 2000 now U.S. Pat. No. 6,772,005, both ofwhich are herein incorporated by reference in their entireties. Thepresent application is also a continuation-in-part of patent applicationSer. No. 10/814,692, filed Mar. 31, 2004, which is also hereinincorporated by reference.

FIELD OF THE INVENTION

The present invention generally relates to implantable medical devicesand more specifically to implantable medical devices for providingcardiac pacing.

BACKGROUND OF THE INVENTION

In a wide variety of commonly employed dual chamber pacing modalities,cross-talk could cause one or more errors. For example, a paced atrialevent may be sensed by a ventricular lead and misinterpreted as aventricular event. This would effectively be far field sensing of anatrial pace. This typically would not be a problem with intrinsic atrialdepolarizations, due to their lower magnitude. Conversely, far fieldsensing of intrinsic R waves or paced ventricular events could likewisebe misinterpreted if sensed by an atrial lead.

To account for such errors, various blanking or refractory periods areemployed such that these events are either not sensed or are simplyignored. In dealing with far field sensing of atrial-paced events,ventricular events that are sensed during a given window following theatrial pace are ignored. Depending upon the application, this window maybe referred to as the atrial blanking period (ABP) or some similarnomenclature. Through clinical application, practitioners havedetermined that if such far field sensing is going to occur, ittypically happens within 80 ms or less of the original event (e.g., theatrial pace). Thus, this window is conservatively set at 80 ms or so,depending upon the specific device or the manufacturer.

In use, such as in a DDD mode, providing this window adequatelyaddresses the cross-talk problem and generally does not createadditional problems. Sometimes genuine intrinsic events will occurduring the window and will also be ignored. For example, a prematureventricular contraction (PVC) is an intrinsic, conducted event but if itfalls within the window it will be ignored.

Whether a far field sense or an intrinsic event, such as a PVC, occursand is ignored, the subsequent action of the device in typical dualchamber modes is to provide a ventricular pace at the expiration of apredetermined interval following the initial atrial event, unlessinhibited. If the ignored event was cross talk, it is certainly possiblethat a subsequent intrinsic ventricular event will occur and inhibit thepace. Alternatively, for any number of reasons no intrinsic event willoccur during the atrial-ventricular interval (AVI) and the ventricularpace is delivered. If the ignored event was a PVC, it is quite likelythat there will not be another intrinsic ventricular event in thecurrent A-A interval and the device will deliver a pacing pulse at theexpiration of the AVI.

Thus, the use of such a window in dual chamber devices is appropriate toprevent cross talk without introducing additional problematic results.As disclosed in the above referenced applications, a mode and/orprotocol is provided that minimizes or greatly reduces ventricularpacing and is referred to as MVP. In summary, MVP tolerates a completecycle (A-A) interval without ventricular activity, in order to promoteintrinsic conduction. In many patients, the conduction pathway is intactbut is delayed beyond the capabilities of traditional dual chamber modetiming. Thus, ventricular pacing is provided when not absolutelynecessary and this is believed to be undesirable.

Various embodiments of MVP are described in greater detail in thereferenced applications, but the mode generally operates by monitoring acomplete cycle for intrinsic conduction. If intrinsic conduction failsand no ventricular event occurs, ventricular pacing is provided in thesubsequent cycle.

The use of the above described window (ABP) presents a challenge to thisminimized or reduced ventricular pacing mode. For example, if true crosstalk occurs and is ignored, subsequent operation continues unhindered.However, if a PVC occurs during this window, it is ignored. Thus, a trueintrinsic ventricular event is being ignored by a mode that bases itsubsequent operation on the presence or absence of intrinsic ventricularactivity during a given cycle. If a PVC occurs during this window it isignored; assuming no other ventricular activity occurs during thisinterval, which is quite possible, the device determines that thecurrent A-A interval is devoid of intrinsic ventricular conduction.Subsequently, the device mode switches or otherwise operates to deliverventricular pacing in a subsequent cardiac cycle and depending upon theembodiment, one or more subsequent cycles. While not in and of itselfharmful, this ventricular pacing is generally not necessary as intactconduction exists. As a result, PVC's may operate to reduce theefficiency of the ventricular minimization or reduction protocolinsomuch as that efficiency is determined to be the reduction orelimination of otherwise unnecessary ventricular pacing. This may simplyresult in a relatively low number of unnecessary ventricular paces.Alternatively, depending upon the specific embodiment of MVP, a seriesof PVCs may be interpreted as a loss of conduction that prevents areturn to the atrial based mode for a longer period of time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a body-implantable device system inaccordance with the present invention, including a hermetically sealeddevice implanted in a patient and an external programming unit.

FIG. 2 is a perspective view of the external programming unit of FIG. 1.

FIG. 3 is a block diagram of the implanted device from FIG. 1.

FIG. 4 is a ladder diagram of the ADI/R operation.

FIG. 5 is a ladder diagram of the committed DDD/R operation in the eventthat the patient develops transient AV block.

FIG. 6 is a ladder diagram that depicts the pacing operation in theevent that the patient develops AV block that persists for more than onecycle.

FIG. 7 is a ladder diagram that depicts a periodic attempt to restorethe ADI/R operation during sustained DDD/R operation.

FIG. 8 is a ladder diagram of the pacing operation in the event that thepatient develops an atrial tachycardia.

FIG. 9 is a flow chart illustrating one embodiment of a mode supervisor

-   -   according to the present invention.

FIGS. 10A-10B are ladder diagrams that illustrate-ventricular senseevents during an ABP.

FIGS. 11A-11B are ladder diagrams that illustrate ventricular senseevents during an ABP, while utilizing a feed back protocol.

FIGS. 12A-12B are ladder diagrams that illustrate ventricular senseevents during an ABP, while utilizing a feed forward protocol.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of an implantable medical device systemadapted for use in accordance with the present invention. The medicaldevice system shown in FIG. 1 includes an implantable device 10—apacemaker in this embodiment—that has been implanted in a patient 12. Inaccordance with conventional practice in the art, pacemaker 10 is housedwithin a hermetically sealed, biologically inert outer casing, which mayitself be conductive so as to serve as an indifferent electrode in thepacemaker's pacing/sensing circuit. One or more pacemaker leads,collectively identified with reference numeral 14 in FIG. 1 areelectrically coupled to pacemaker 10 in a conventional manner and extendinto the patient's heart 16 via a vein 18. Disposed generally near thedistal end of leads 14 are one or more exposed conductive electrodes forreceiving electrical cardiac signals and/or for delivering electricalpacing stimuli to heart 16. As will be appreciated by those of ordinaryskill in the art, leads 14 may be implanted with its distal end situatedin the atrium and/or ventricle of heart 16.

Although the present invention will be described herein in oneembodiment which includes a pacemaker, those of ordinary skill in theart having the benefit of the present disclosure will appreciate thatthe present invention may be advantageously practiced in connection withnumerous other types of implantable medical device systems, and indeedin any application in which it is desirable to provide the preferredADI/R pacing mode (i.e., the MVP modality), as may occur in ICDs and thelike.

Also depicted in FIG. 1 is an external programming unit 20 fornon-invasive communication with implanted device 10 via uplink anddownlink communication channels, to be hereinafter described in furtherdetail. Associated with programming unit 20 is a programming head 22, inaccordance with conventional medical device programming systems, forfacilitating two-way communication between implanted device 10 andprogrammer 20. In many known implantable device systems, a programminghead such as that depicted in FIG. 1 is positioned on the patient's bodyover the implant site of the device (usually within 2- to 3-inches ofskin contact), such that one or more antennae within the head can sendRF signals to, and receive RE signals from, an antenna disposed withinthe hermetic enclosure of the implanted device or disposed within theconnector block of the device, in accordance with common practice in theart.

FIG. 2 is a perspective view of programming unit 20 in accordance withthe presently disclosed invention. Internally, programmer 20 includes aprocessing unit (not shown in the Figure) that in accordance with thepresently disclosed invention is a personal computer type motherboard,e.g., a computer motherboard including an Intel Pentium 3 microprocessorand related circuitry such as digital memory. The details of design andoperation of the programmer's computer system will not be set forth indetail in the present disclosure, as it is believed that such detailsare well known to those of ordinary skill in the art.

Referring to FIG. 2, programmer 20 comprises an outer housing 60, whichis preferably made of thermal plastic or another suitably rugged yetrelatively lightweight material. A carrying handle, designated generallyas 62 in FIG. 2, is integrally formed into the front of housing 60. Withhandle 62, programmer 20 can be carried like a briefcase.

An articulating display screen 64 is disposed on the upper surface ofhousing 60. Display screen 64 folds down into a closed position (notshown) when programmer 20 is not in use, thereby reducing the size ofprogrammer 20 and protecting the display surface of display 64 duringtransportation and storage thereof.

A floppy disk drive is disposed within housing 60 and is accessible viaa disk insertion slot (not shown). A hard disk drive is also disposedwithin housing 60, and it is contemplated that a hard disk driveactivity indicator, (e.g., an LED, not shown) could be provided to givea visible indication of hard disk activation.

As would be appreciated by those of ordinary skill in the art, it isoften desirable 30 to provide a means for determining the status of thepatient's conduction system. To accomplish this task and providesuitable ECG tracings, programmer 20 is equipped with external ECG leads24.

In accordance with the present invention, programmer 20 is equipped withan internal printer (not shown) so that a hard copy of a patient's ECGor of graphics displayed on the programmer's display screen 64 can begenerated. Several types of printers, such as the AR-100 printeravailable from General Scanning Co., are known and commerciallyavailable.

In the perspective view of FIG. 2, programmer 20 is shown witharticulating display screen 64 having been lifted up into one of aplurality of possible open positions such that the display area thereofis visible to a user situated in front of programmer 20. Articulatingdisplay screen is preferably of the LCD or electro-luminescent type,characterized by being relatively thin as compared, for example, acathode ray tube (CRT) or the like.

As would be appreciated by those of ordinary skill in the art, displayscreen 64 is operatively coupled to the computer circuitry disposedwithin housing 60 and is adapted to provide a visual display of graphicsand/or data under control of the internal computer. Programmer 20described herein with reference to FIG. 2 is described in more detail inU.S. Pat. No. 5,345,362 issued to Thomas J. Winkler, entitled “PortableComputer Apparatus With Articulating Display Panel,” which patent ishereby incorporated herein by reference in its entirety. The MedtronicModel 9790 programmer is the implantable device-programming unit withwhich the present invention may be advantageously practiced.

FIG. 3 is a block diagram of the electronic circuitry that makes uppulse generator 10 in accordance with the presently disclosed invention.As can be seen from FIG. 3, pacemaker 10 comprises a primary stimulationcontrol circuit 20 for controlling the device's pacing and sensingfunctions. The circuitry associated with stimulation control circuit 20may be of conventional design, in accordance, for example, with what isdisclosed U.S. Pat. No. 5,052,388 issued to Sivula et al., “Method andapparatus for implementing activity sensing in a pulse generator.” Tothe extent that certain components of pulse generator 10 areconventional in their design and operation, such components will not bedescribed herein in detail, as it is believed that design andimplementation of such components would be a matter of routine to thoseof ordinary skill in the art. For example, stimulation control circuit20 in FIG. 3 includes sense amplifier circuitry 24, stimulating pulseoutput circuitry 26, a crystal clock 28, a random-access memory andread-only memory (RAM/ROM) unit 30, and a central processing unit (CPU)32, all of which are well-known in the art.

Pacemaker 10 also includes internal communication circuit 34 so that itis capable communicating with external programmer/control unit 20, asdescribed in FIG. 2 in greater detail.

With continued reference to FIG. 3, pulse generator 10 is coupled to oneor more leads 14 which, when implanted, extend transvenously between theimplant site of pulse generator 10 and the patient's heart 16, aspreviously noted with reference to FIG. 1. Physically, the connectionsbetween leads 14 and the various internal components of pulse generator10 are facilitated by means of a conventional connector block assembly11, shown in FIG. 1. Electrically, the coupling of the conductors ofleads and internal electrical components of pulse generator 10 may befacilitated by means of a lead interface circuit 19 which functions, ina multiplexer-like manner, to selectively and dynamically establishnecessary connections between various conductors in leads 14, including,for example, atrial tip and ring electrode conductors ATIP and ARING andventricular tip and ring electrode conductors VTIP and VRING, andindividual electrical components of pulse generator 10, as would befamiliar to those of ordinary skill in the art. For the sake of clarity,the specific connections between leads 14 and the various components ofpulse generator 10 are not shown in FIG. 3, although it will be clear tothose of ordinary skill in the art that, for example, leads 14 willnecessarily be coupled, either directly or indirectly, to senseamplifier circuitry 24 and stimulating pulse output circuit 26, inaccordance with common practice, such that cardiac electrical signalsmay be conveyed to sensing circuitry 24, and such that stimulatingpulses may be delivered to cardiac tissue, via leads 14. Also not shownin FIG. 3 is the protection circuitry commonly included in implanteddevices to protect, for example, the sensing circuitry of the devicefrom high voltage stimulating pulses.

As previously noted, stimulation control circuit 20 includes centralprocessing unit 32 which may be an off-the-shelf programmablemicroprocessor or micro controller, but in the present invention is acustom integrated circuit. Although specific connections between CPU 32and other components of stimulation control circuit 20 are not shown inFIG. 3, it will be apparent to those of ordinary skill in the art thatCPU 32 functions to control the timed operation of stimulating pulseoutput circuit 26 and sense amplifier circuit 24 under control ofprogramming stored in RAM/ROM unit 30. It is believed that those ofordinary skill in the art will be familiar with such an operativearrangement.

With continued reference to FIG. 3, crystal oscillator circuit 28, inthe presently preferred embodiment a 32,768-Hz crystal controlledoscillator provides main timing clock signals to stimulation controlcircuit 20. Again, the lines over which such clocking signals areprovided to the various timed components of pulse generator 10 (e.g.,microprocessor 32) are omitted from FIG. 3 for the sake of clarity.

It is to be understood that the various components of pulse generator 10depicted 10 in FIG. 3 are powered by means of a battery (not shown) thatis contained within the hermetic enclosure of pacemaker 10, inaccordance with common practice in the art. For the sake of clarity inthe Figures, the battery and the connections between it and the othercomponents of pulse generator 10 are not shown.

Stimulating pulse output circuit 26, which functions to generate cardiacstimuli 15 under control of signals issued by CPU 32, may be, forexample, of the type disclosed in U.S. Pat. No. 4,476,868 to Thompson,entitled “Body Stimulator Output Circuit,” which patent is herebyincorporated by reference herein in its entirety. Again, however, it isbelieved that those of ordinary skill in the art could select from amongmany various types of prior art pacing output circuits that would besuitable for the purposes of practicing the present invention.

Sense amplifier circuit 24, which is of conventional design, functionsto receive electrical cardiac signals from leads 14 and to process suchsignals to derive event signals reflecting the occurrence of specificcardiac electrical events, including atrial contractions (P-waves) andventricular contractions (R-waves). CPU provides these event-indicatingsignals to CPU 32 for use in controlling the synchronous stimulatingoperations of pulse generator 10 in accordance with common practice inthe art. In addition, these event-indicating signals may becommunicated, via uplink transmission, to external programming unit 20for visual display to a physician or clinician.

Those of ordinary skill in the art will appreciate that pacemaker 10 mayinclude numerous other components and subsystems, for example, activitysensors and associated circuitry. The presence or absence of suchadditional components in pacemaker 10, however, is not believed to bepertinent to the present invention, which relates primarily to theimplementation and operation of communication subsystem 34 in pacemaker10, and an associated communication subsystem in external unit 20.

FIG. 4 is a ladder diagram of the ADI/R operation, specifically a MarkerChannel® Diagram. With the help of the (pre-2002) NBG Code, one familiarwith the state of the art will be able to discern that the letter in thefirst position (A) means that the pacemaker (or other implanted device)will pace the atrium in the absence of an atrial sensed event. Thesecond letter (D) implies that the pacemaker will sense in dualchambers, that is, both the atrial and ventricular chambers. The thirdletter (I) means that, upon sensing in either chamber, pacing will beinhibited in that specific chamber. The final letter, R, implies thatthe device may be rate responsive, that is, altering the atrial rate inresponse to an artificial sensor, such as a Piezo-electrical crystal,accelerometer, minute ventilation, etc.

The operation of the preferred ADI/R mode is depicted in the ladderdiagram as follows. Atrial paced (or sensed) event 1 initiates anon-programmable, auto 15 adjusting (e.g., 100-150 millisecond) blankingperiod 4, followed by auto-adjusting atrial sensitivity (not shown).Sensing circuitry (see FIG. 3) determines if and when ventricular sensedevent 2 has occurred. If detected, timing circuitry (see FIG. 3)initiates VA interval 9. Other timing, blanking periods, and refractoryperiods serve the following purposes. A programmable ventricularblanking period 8 prevents sensing of atrial pace 1 on the ventricularchannel, sometimes termed “cross-talk.” Ventricular sensed event 2starts a 120 millisecond post ventricular atrial blanking (PVAB) period6, followed by auto-adjusting atrial sensitivity. PVAB 6 serves thepurpose of preventing sensing of the R-wave or T-wave on the atrialchannel, termed “far-field R-wave sensing.” Ventricular sensed event 2also starts 100 millisecond ventricular blanking 7 followed byauto-adjusting ventricular sensitivity. This period serves the purposeof preventing sensing of the ventricular output pulse or the ventriculardepolarization itself. Repolarization, or T-wave 3, follows R-wave 2.Ventricular event 2 detected by sensing circuitry (see FIG. 3) sendssignal to timing circuitry to start VA interval 9, leading to the nextatrial pacing cycle. Two R-R intervals are depicted in FIG. 4. Asdescribed in more detail hereinbelow, an ARP may have a nominal value ofapproximately seventy percent (70%) of a single preceding R-R interval(in a beat-to-beat implementation) or of a series of preceding R-Rintervals.

Taking into account that this mode would be used primarily with SickSinus patients who have full or some degree of intact AV conduction,this type of operation as depicted for the ADI/R mode is something theclinician or physician would expect to occur. In the presence ofrelatively reliable intact AV conduction the pacemaker will maintain theADI/R operation/mode. Sensed ventricular events would occur in the vastmajority of cardiac cycles (that is, PQRST). FIG. 5 teaches what willoccur should the patient develop transient AV block for one or a fewcardiac cycles.

FIG. 5 is a ladder diagram of the DDI/R operation in the event that thepatient experiences a PVC, in one embodiment. The purpose of the DDI/Roperation is to maintain ventricular support (i.e., help the patientrecover sufficient cardiac output following the PVC). Briefly stated,the implanted device mode switches from the preferred ADI/R to the DDI/Rfor in response to a detected PVC for at least one cardiac cycle.

The timing of the DDI/R is as follows. In the DDI/R mode (third pacingcycle, labeled DDI/R), AV interval 5 is set to a short period (e.g., 80milliseconds), following the paced P-wave due to the presence of a PVCbetween the second and third atrial paced events. The purpose of thisshort AV interval 5 is intended to suppress competition betweenventricular pacing pulse culminating in paced R-wave 13 and anypotential intrinsic R-wave with a delayed conduction from the previouspaced atrial event. Assuming the presence of such an intrinsic R-wave,the timing of the ventricular output pulse would normally result in aventricular pacing pulse falling into the absolute refractory period ofthe intrinsic, conducted R-wave, resulting in a psuedo-fusion beat (notshown). This operation is intended to prevent the onset of a ventriculartachycardia, should the ventricular pacing pulse fall into the relativerefractory period of the ventricle, commonly called “pacing on T”phenomenon. In this respect, the reader is again cautioned that thedrawings do not necessarily reflect actual or practical timing, but areintended to illustrate the notion of a mode switch (to DDI/R) followinga PVC.

With respect to the foregoing, in one form of the invention, if the Apencroaches on the preceding Vs (e.g. within 300 msec) for more thanabout four 30 depolarization events (e.g., consecutive beats), then thepacing rate is decreased. In effect, this creates a dynamic upper sensorrate. Thus, the present invention addresses an anticipated concerns withregard to the MVP modality providing relatively short VS-AP intervals.Such intervals could cause disadvantageous patient symptoms and may alsohave a negative heart remodeling effect. To counter these issues the MVPmodality can operate such that after a V-Sense event (Vs), a scheduledA-Pace (Ap) event is delayed until some pre-defined interval expires.This aspect of the MVP modality is somewhat similar to upper trackingrate (UTR) hold off or non-competitive atrial pacing (NCAP) hold-offexcept that it is based on an A-Pace (Ap) event following a V-Sense(Vs). This results in the atrium being paced at a slightly lower ratethan intended which may create issues that are known to exist withrespect to so-called atrial overdrive pacing algorithms. This aspect ofthe MYP modality is preferably implemented in hardware (just like UTRand NCAP) primarily because of the critical timing involved.

In order to prevent adverse hemodynamics that may result from atrialpacing soon (e.g. within 250 msec) after a ventricular sense (i.e.Vp-As) while in the preferred ADI/R mode of pacing, one option is to(and subsequently limit for a period of time (e.g. one hour) the sensordriven pacing rate in the event of continuous cycles (e.g. 4-8consecutive) of atrial pacing within a programmable interval (e.g. 250msec) of the preceding R-waves. For example, such a dynamic upper ratelimit is preferably set so that the Vs-Ap interval does not decrease toless than about 300 ms.

Continuing with the timing in FIG. 5, paced R-wave 13 starts a 120millisecond ventricular blanking period 7, followed by auto adjustingventricular sensitivity (not shown). Paced R-wave 13 also starts a 120millisecond PVAB 6 followed by auto adjusting atrial sensitivity (notshown). Assuming the transient AV block self-corrects and a sensedR-wave is detected in response to the ventricular pace (Vp), thepreferred ADI/R resumes with the next paced or sensed P-wave, as isdepicted in FIG. 4.

FIG. 6 is a ladder diagram that depicts the pacing operation in theevent that the patient develops AV block for more than one cycle. Notethat according to the preferred embodiment of the present invention, asingle missed beat (i.e., no Vs) will not by itself cause a mode switch,particularly if relatively reliable AV conduction is present. Followinga mode switch to DDI/R, VA interval 9 times out, resulting in atrialpaced event 1. A very long (e.g. 400 millisecond or up to approximately70% of the median V-V interval) 17 may be used in an attempt to promotenative AV conduction (or a Vp stimulus may be withheld) as furtherdescribed hereinbelow. If, however, AV interval 17 is not interrupted bya sensed, intrinsic R-wave, as is depicted in the first cycle (labeledADI/R), the pacemaker immediately switches to the DDD/R mode. In theevent that a sensed, intrinsic R-wave does occur, the device reverts tothe ADI/R operation (not shown). The DDD/R operation, with theprogrammed AV interval, will be sustained until a sensed, intrinsicR-wave is detected, as further described herein. Periodic attempts toforce restoration of the ADI/R operation may be performed (as depictedin FIG. 7). Mode switching to the DDI/R mode may occur in the event thata PVC is detected and in the event that that an atrial tachycardia isdetected a mode switch to DDD/R pacing is preferred.

FIG. 7 is a ladder diagram that depicts a periodic attempt to restorethe ADI/R operation during sustained DDD/R operation. As mentioned, theDDD/R mode may become the sustained mode of operation in the event thatthe patient develops a prolonged AV block, such as might occur withrate-dependent AV block or if the AV conduction become relativelyunreliable. In such cases, the device may be programmed to revert toADI/R 1 after a programmable number of DDD/R cycles. Then, the devicelooks for a ventricular sensed event, e.g., at 23 following atrial pace1. In the event that a sensed, intrinsic R-wave is detected, the ADI/Roperation is immediately resumed. In the absence of a ventricular sensedevent, the device continues to operate in the DDD/R mode, as indicatedin the third cycle of FIG. 7.

FIG. 8 is a ladder diagram of the pacing operation in the event that thepatient 20 develops an atrial tachycardia. A sick sinus patient oftenhas episodes of atrial tachycardia, atrial flutter, or atrialfibrillation. During these episodes, the pacing operation must be setsuch that the ventricular pacing rate will neither be synchronized tothe fast atrial rate nor so slow as to cause symptoms. Preferably duringepisodes of AT, the atrial-based pacing ends and a DDD/R (or DDI/R)pacing mode is employed.

In FIG. 5 it was noted that the device, while operating in/mode also iswell suited for pacing in the presence of an atrial tachycardia becauseit will not allow ventricular synchronization to a fast atrial rate norwill it allow the ventricular pacing rate to go below the programmedlower rate. Therefore, when an atrial tachycardia does occur, as shownin FIG. 8, fast atrial sensed events 27 without a conducted ventricularevent have no effect on ventricular timing 9. Since there is noventricular event, the operation immediately switches to the DDI/R mode.In the presence of an atrial tachycardia, the V-V interval 9 times outso that paced R-wave 8 will occur at the faster of the programmed lowerrate or sensor-indicated rate in the DDI/R mode. The operation depictedin FIG. 8 will continue so long as the atrial tachycardia persists. Upontermination of the atrial tachycardia, the preferred ADI/R will resumeas shown in FIG. 4 or 7, depending on how the heart recovers from theatrial tachyarrhythmia. If the atrial tachyarrhythmia terminatesabruptly, the prompt restoration of the ADI/R mode may take place (seeFIG. 4). If, however, the atrial tachyarrhythmia “cools down” slowly,there may be a period of DDD/R pacing with periodic attempts to restoreADI/R pacing as shown in FIG. 7.

In contrast to a majority of the foregoing, and with general referenceto FIG. 9, the MVP modality includes one or more of the followingaspects.

Adaptive Atrial Refractory Period (ARP)

According to the initial definition of the preferred ADI/R modality, arate-adaptive ARP is employed in order to distinguish physiologic atrialevents from non physiologic events. According to a preferredimplementation, an adaptive ARP is employed and defined as a fixedpercentage of the physiologic interval (P1). One preferred method ofdetermining the P1 is based on the ventricular rate as determined by themedian R-R interval for the preceding 12 ventricular events (regardlessif such events are sense- or pace-type events). Specifically the medianvalue is determined algorithmically as the seventh longest interval ofthe preceding 12 R-R (i.e., V-R, RV, or V-V) intervals. Therefore,recalculation of the P1 occurs following event ventricular event as anew interval is added to a 12 beat accumulator (e.g., temporary memorystructure) and the oldest is eliminated according on a FIFO (first-in,first-out) basis. Of course, a beat-to-beat instantiation may be used inlieu of the multi-beat techniques described herein.

The preferred implementation defines ARP as a programmable, fixedpercentage of the P1. A suggested default value is seventy percent (70%)of the R-R interval (either a calculated value—such as a median value—ora beat-to-beat value derived from a prior R-R interval). Thereby,intrinsic atrial events that occur at regular intervals (consistent witha patient's current physiologic state) that fall outside of the ARP canbe defined as physiologic while those within the ARP can be assumed torepresent noise or are otherwise not physiologic. Alternatively, the ARPcan be implemented as an adaptive approach with a fixed, absolute timeperiod (i.e., fixed period of time maintained for the remainder of theP1). The philosophy behind the latter approach is for avoidance ofatrial competitive pacing during the physiologic refractory period ofthe atrium. One possible downside of a fixed (e.g., 300 ms) alert periodoutside of the ARP, however, is the increased risk of misclassifyingnon5 physiologic atrial events as physiologic.

Mode Supervisor:

The Wenckebach supervisor (as briefly described previously) has beenrenamed the “mode supervisor” because the mode supervisor can control awide range of operations related to mode changes. The primary intent ofthe mode supervisor is to monitor a patient's atrioventricular statusand intervene when necessary by invoking sustained mode-switches toconventional modes of pacing (i.e. DDD/R and DDI/R). According to thepreferred implementation, the mode supervisor defines unreliable AVconduction according to a Wenckebach pattern with definition of acritical AV conduction acceptance ratio to discriminate betweentolerable (or “relatively reliable”) AV conduction states fromintolerable (or “relatively unreliable”) AV conduction states. Forinstance, an AV conduction acceptance ratio of 4:3 allows preferredADI/R operation to persist as long as there are at least threeventricular events for every four physiologic atrial events. Should AVconduction falter such that the ratio of A to V events falls below thepre-defined acceptance ratio, a sustained switch to conventional DDD/Rpacing will occur. Importantly, atrial events classified asnon-physiologic (i.e. within the ARP) are not accounted for in thecalculation of the A:V ratio. Thereby, inappropriate mode-switches toDDD/R are avoided in the presence of frequent non-conducted prematureatrial contractions (PAC).

Upon invoking DDD/R pacing in the presence of unreliable AV conduction,the mode supervisor immediately assumes the role of striving to restorepreferred ADI/R pacing. Since it is known that AV conduction diseasetypically progresses gradually with brief manifestations of high degreeblock expected in the early stages of disease progression, the modesupervisor will attempt to restore preferred ADI/R operation followingonly a brief episode of new onset DDD/R pacing. According to thepreferred operation, the first reattempt to reveal intact AV conductionand to restore ADI/R pacing will occur only after a short period of time(e.g., one minute) of DDD/R pacing. Should ADL'R restoration fail,reattempts will be attempted at 2, 4, 8, 16 and 32 minutes andsubsequently at 1, 2, 4, 8, 12 and 24 hours. Of course, other timingsequences may be used, both periodic and aperiodic (as well as local andremote clinician- or patient-activated atrial-based pacing initiation).

The algorithm used to search for intact AV conduction and restore ADI/Ris defined according to one of two options. The first option is tosimply withhold a ventricular pace stimulation during DDD/R operation.In the event that a ventricular sense follows the physiologic atrialevent during which ventricular pacing was withheld, ADI/R pacing isresumed. Otherwise, DDD/R pacing continues with subsequent reattemptsaccording to a schedule or by way of manual activation (as specifiedabove). The second option searches for intact AV conduction involvesextending the AV delay during DDD/R pacing to a pre-designated AVconduction [search] interval (AVCI). For instance, with an AVCI of 400ms, the AV delay is extended to 400 ms following a physiologic atrialevent (sensed or paced). In the event that the AV interval isinterrupted by a ventricular sense, thereby preempting the ventricularpace in DDD/R operation, the mode supervisor reverts to ADI/R operation.Otherwise, a ventricular pace is delivered upon the expiration of theAVCI interval and DDD/R operation resumes with reattempts according tothe schedule (or with manual activation) as described above.Importantly, in the event of failed conduction and ventricular pacingduring these AV conduction search methods, an extended post-ventricularatrial refractory period (PVARP) in invoked following the AVCI in orderto guard against the possibility of retrograde conduction initiating apacemaker mediated tachycardia.

A third responsibility of the mode-supervisor is to recognize sustainedpathologic atrial rhythms and to invoke sustained mode-switching toDDI/R pacing for the duration of the atrial tachyarrhythmia (AT). It isexpected that the defining AT criteria will be consistent with that usedby conventional pacing modes (e.g. 4 of 7 short A-A intervals) and thatmode-switching operation will not be unique to the minimum ventricularpacing (MYP) modality and therefore is not further described in thisdisclosure. The uniqueness of the implementation within MVP lies in thepossibility that mode-switching to DDI/R will occur either from theADI/R or DDD/R operating states. The inventors believe that the notionof switching to/from DDI/R is novel, and although not practicallynecessary as ADI/R is not an atrial tracking mode, there may be somemerit to switching directly to DDI/R in order to avoid an inadvertentswitch to DDD/R in the event of transient conduction block during an AFevent. Moreover, a sustained switch to DDI/R may be justified in orderto provide some degree of rate-regularization during AF with anirregular ventricular response.

A fourth responsibility of the mode supervisor is to monitor for rapidrepeated switches between preferred ADI/R and DDD/R pacing modes. If thedevice repeatedly switches back and forth between these modes everyminute or every two minutes (e.g., or other relatively short period oftime) the mode supervisor can suspend testing for AV conduction andallow the device to remain in DDD/R pacing, for example by setting theAV conduction testing interval to some number of hours (e.g. 2, 4, 8,16). The number of repeated mode changes required to trigger suchbehavior remains to be determined and may be programmable.

A fifth responsibility of the mode supervisor is to monitor for repeatedfailed AV conduction tests at maximal test duration. So for example, ifseven straight tests for AV conduction fail at 16 hour intervals, themode supervisor can suspend AV conduction testing and the device canthen remain in the DDD/R mode indefinitely.

A sixth responsibility of the mode supervisor involves suspending AVconduction testing based on physiologic parameters (rather thanindefinitely terminating searches or simply suspending for a fixednumber of hours or other period of time). For example, the modesupervisor can monitor heart rate and recognize that repeated switchingback and forth between preferred ADI/R and DDD/R is associated with highheart rates (HR) or activity, and suspend AV conduction testing untilthe HR returns below a preset or dynamically set HR threshold. Similarfunctionality can be implemented in the case of rapid repeated switchingassociated with just low heart rates.

A seventh responsibility of the mode supervisor relates to varying thetolerated Wenckebach threshold dependent on the time of day or a signalfrom a sleep indicator. For example, in patients with known incidence ofWenckebach during sleep, the supervisor changes the threshold totolerate more severe Wenckebach at that time in response to a positiveindication that the patient has entered a sleep state or simply as amatter of timing (e.g., increase Wenckebach tolerance during expectedsleep time of the patient).

An eighth responsibility of the mode supervisor involves maintaining arecord of the sensor driven atrial paced rate at which the Wenckebachthreshold was exceeded during ADI/R operation (thereby causing a modeswitch to DDD/R). Subsequently, the upper sensor rate is thus restrictedto not encourage high rate sensor driven pacing above rates at whichreliable AV conduction does not exist. This operation, in essence, is adynamic upper sensor rate that adapts according to information obtainedduring mode excursions from ADI/R to DDD/R.

A ninth responsibility of the mode supervisor relates to controlling thepacing mode of an ICD following delivery of a defibrillation therapy tothe patient (i.e., high voltage shock delivery). In this aspect of theinvention, the mode supervisor initiates ADI/R pacing with a DDIsequence, or in the ADI/R mode at a premature timing interval followingdelivery of a defibrillation therapy (i.e., a high voltage shock) inorder to prevent a significant delay in delivery of a ventricular pace(Vp) in the event of transient post-shock AV block. Alternatively, apreferred option favors DDD/R pacing and delays resumption of ADI/Rpacing for a pre-specified period of time following delivery of such ahigh voltage defibrillation shock.

PVC Response

According to ADI/R operation, premature ventricular contractions (PVC)will 20 not alter the timed delivery of the ensuing atrial pace. Sincethis can conceivably result in a closely coupled conducted ventricularevent due to atrial pacing coincident or soon following a PCV, theinventors decided to deviate from ADI/R operation in this circumstance,in some embodiments, and effectively operate in a DDI/R modality. Indoing so, following a PVC event the ensuing atrial pace is delayed andscheduled according the operating AV delay (preferably equal to the P1minus 80 ms). In addition to providing more appropriate rhythm responsesduring bradycardia pacing operation interrupted by PVCs, the addedadvantage of having this PVC response is that asynchronous atrial pacingis avoided during runs of ventricular tachycardia. This has especiallyimportant implications for tachyarrhythmia control devices, whichtypically require consecutive detected VT intervals, as withholdingatrial pacing during VT also removes the potentially interferingcross-chamber ventricular blanking periods that occur with atrialpacing.

Various aspects of certain embodiments of the present invention can beimplemented using executable software code and/or operational parameterssaved by (or downloaded to) a medical device. Such a device may bedisposed in vivo and later programmed according to the invention or maybe programmed prior to implantation (e.g., using firmware that may bereprogrammed or modified using telemetry techniques and the like). Thisis in contrast to a beat-to-beat implementation of the invention, whichwould preferably be implemented in hardware as understood by those ofskill in the art. However, the present invention is not limited to onlyfirmware or hardware implementations; indeed, the present invention maybe implemented in a hybrid or combined in any desirable manner usingdevice programming techniques known and used in the art. For clarity,however, the inventors specifically provide and herein claim abeat-to-beat instantiation of the present invention wherein theoperation of the MVP modality is invoked for every beat on abeat-to-beat basis.

Referring to FIGS. 10A and 10B, the above referenced MVP protocols willgenerally handle cross talk and PVC's in the following manner. In FIG.10A, an atrial pace is delivered during interval 1 and an atrialblanking period ABP or similar window is defined. Typically, such awindow is on the order of about 80 ms but may be defined as desired. Anyventricular event, whether intrinsic or far-field is ignored if itoccurs during the ABP. Thus, during interval 2 a PVC occurs within theABP and is ignored. Despite there having been an intrinsic, conductedventricular event the protocol considers interval 2 to be devoid ofventricular activity. Depending upon the embodiment of MVP, ventricularpacing is provided in interval 3 and in subsequent intervals until aconduction check occurs, a ventricular pace is inhibited, or anotherevent occurs to switch the device back to the atrial based mode. Inother words, despite there being intrinsic conduction present,ventricular pacing is provided; thus, reducing the efficiency of MVP ineliminating as much ventricular pacing as possible.

FIG. 10B illustrates cross talk or far field sensing of the atrial pacethat is sensed by the ventricular lead and indicated on the ventricularchannel during the ABP. As these events are ignored, this will have noeffect on the MVP. Furthermore, the ABP is successful in preventing suchcross talk from being misinterpreted. In other words, in this instancethe ABP is performing its intended function and does not negativelyaffect the protocol for minimizing or reducing ventricular pacing. Thus,while cross-talk is appropriately handled, PVC's or other intrinsicventricular events that occur during the ABP reduce the efficiency.

To avoid this reduction in efficiency, the present invention provides“feed back” or “feed forward” cross talk filtering protocols that areimplemented with MVP. It is understood that there are many embodimentsof MVP and that each will tolerate or react to missed ventricular beatsin a variety of ways. The following description is meant to apply to anyof these embodiments, though each variation is not separately describedin detail.

FIG. 11A illustrates an embodiment using a “feed back” cross talkfiltering protocol to address this situation. Under the “feed back”protocol, an event sensed in the ABP during an interval is classified asinvalid, and hence ignored. This classification information is stored inhardware, software, firmware, or memory as a marker or other indicator.The ABP of the subsequent cycle (e.g., 3) is monitored. If there is asimilar sense during this ABP, then the earlier classification ismaintained. If there is no similar sense during the ABP then the earlierevent is “reclassified” as a valid ventricular event and treated as suchby the MVP algorithm. For example, in interval 2, a PVC occurs duringthe ABP. Initially, this is classified as invalid. During the nextinterval 3, there is no sensed event during the ABP and at theexpiration of the ABP the previous PVC is reclassified as valid. Thus,interval 2 is indicated to have intrinsic ventricular conduction as ofthe expiration of the ABP in interval 3. As such, no ventricular pacingis provided during interval 3. From this point forward, operationcontinues normally under MVP.

In practice, this reclassification can be implemented in any number ofways. For example, if MVP is utilizing actual mode switching, theappropriate mode switch (assuming no ventricular sense in interval 2)could occur at the atrial pace for interval 3 with a subsequent modeswitch (either actual or effective) at the expiration of the ABP.Alternatively, the mode switch to the dual chamber mode could be set tooccur only after the ABP if there is no reclassification. Additionally,utilizing flags would include setting an appropriate flag after the ABPwith the subsequent functionality resulting. As all of this occurs inthe interval of interest, various ad hoc or single interval steps may betaken to address the implications of the short timeline leading to areturn to normal MVP operation.

FIG. 11B illustrates a case where cross talk occurs during the ABP ofinterval 2. Again, since this is the first occurrence this ventricularsense is classified as invalid. During the subsequent ABP which occursin interval 3, far field sensing will most likely occur again and isillustrated as such. Therefore, the ventricular sense of interval 2 isnot reclassified, but rather remains invalid and ignored. MVP willfunction based upon whether or not ventricular events are sensed outsideof the ABP.

Under extremely rare circumstances, this may lead to short termanomalous behavior that is either tolerated or in some embodimentsaddressed. It is conceivable (FIG. 11B) that far field sensing occursonly during interval 2 and the sensed event in interval 3 is actually aPVC or other intrinsic event. One option is to simply ignore thisextremely unlikely scenario and simply treat the PVC as a far fieldsense. Another option would be to scrutinize the timing within the ABPof the various ventricular sensed events. Far field sensing should havepredictable timing; thus, a PVC may fall within the ABP but havesufficiently different timing to distinguish this event so as to eitherreclassify the event of interval 2 (erroneously) or cause the event ofinterval 3 to be evaluated differently in intervals 4 and beyond. Inother words, timing may be used to further distinguish ventricularsenses during the ABP from binary results to more analytical results todistinguish between events when cross talk and PVC's are occurring.Furthermore, if far field sensing occurs along with and is separatelydistinguishable from a PVC during the ABP, this could also be used toindicate that intrinsic conduction is present along with cross talk.

FIGS. 12A and 12B illustrate a “feed forward” cross talk filteringprotocol for addressing sensed events during the ABP. In FIG. 12A, a PVCoccurs during the ABP of interval 2. In this protocol, the firstoccurrence of such an event is classified as valid; in other words, itis considered an intrinsic, conducted event. Moving forward in time, MVPwill behave normally from that point on. Thus, any sensed event on theventricular channel is treated as a valid by MVP, regardless of when itoccurs during the interval. While providing the desired results in theefficiency of MVP, this alone would simply be equivalent to eliminatingthe ABP and would leave open the problems associated with far fieldsensing or cross talk.

FIG. 12B illustrates how this problem is addressed. Here, theventricular sense during the ABP of interval 2 is cross talk. Asindicated, this occurrence is classified or considered as a validventricular event. Thus, whether or not an intrinsic ventriculardepolarization occurs during interval 2, the MVP protocol acts as if ithas. The classification of this event during the ABP as valid is “fedforward” and affects subsequent senses during the ABP. In this example,cross talk is sensed during the ABP of interval 3. The “fed forward”indicator has been toggled and this sense during the ABP of interval 3is classified as invalid. Assuming cross talk is sensed duringsubsequent ABPs, those events will also be considered invalid. Thus,only a ventricular sense occurring outside of the ABP will satisfy theMVP protocol in interval 3 and beyond.

This may be a one-time toggle with an assumption that once cross talk issensed, it will remain an issue until manually changed or corrected.Alternatively, the protocol may include provisions to reset itself if apredetermined number of intervals transpire without sensed activityduring the ABP. This will automatically address intermittent cross talk.The predetermined number of intervals may be fixed or vary. That is, forthe first occurrence, the system may reset after a few intervals but ifcross talk is observed with some frequency, then attempts to reset aremade less frequent and/or eliminated.

Similarly to the “feed back” protocol, the “feed forward” protocolconceptually permits some extremely unlikely anomalous behavior. First,PVCs may occur in consecutive cycles and be misinterpreted as crosstalk. One solution is the automatic and/or periodic reset indicatedabove. Another option is to monitor the timing of the event within theABP. True far field sensing should be somewhat stable and predictable.Thus, if the sensed events vary in time within the ABP by more that apredetermined percentage, these events may be reclassified as PVCs or atleast subjected to a higher level of evaluation. Similarly, both farfield sensing and PVCs could be present. This is simply solved byapplying the above classifications, but if multiple events occur duringa single ABP consider one to be an intrinsic ventricular event, subjectto the remaining protocol parameters.

An even more remote scenario could occur in that a patient has intrinsicconduction and no cross talk for some period of time, e.g., up throughinterval 1 of FIG. 12B. Then, during the ABP of interval 2, a far fieldsense occurs. As indicated, with the “feed forward” protocol, this farfield sense is classified as valid. In the same interval, the patient'sconduction fails and there is no intrinsic ventricular activity.However, since the far field sense is classified as valid, the MVPprotocol considers interval 2 to have a ventricular event. Subsequently,in interval 3, an atrial pace AP is delivered. During this ABP therecould be another far field sense, which is classified as invalid due tothe “feed forward” indicator. Again, assuming conduction block hasoccurred there is no intrinsic conduction; thus, there is no ventriculardepolarization. At the conclusion of interval 3, the MVP protocol willrecognize the absence of conduction and respond accordingly. Typically,this means providing a ventricular pace. However, in some embodiments,the mode supervisor MS may seek to have more than one interval out of apredetermined number of intervals without ventricular activity. Since,interval 3 was the first interval considered to be devoid of ventricularactivity, the mode supervisor MS could permit interval 4 to transpirewithout ventricular pacing. Thus, it would be conceivable to have two oreven three intervals without ventricular activity.

There are several ways to address this issue. The first is to realizethat the likelihood of initiating far field sensing and loosingintrinsic conduction during the same cycle are miniscule. Even if thiswere to result, the effect on the patient would be tolerable and pacingwould result in the third or fourth interval. Thus, while conceptuallypossible, the practical likelihood and results render this issueessentially moot.

To prevent even the chance of this occurring, certain steps may betaken. For example, only certain embodiments of the mode supervisor MSwill permit a situation where three intervals could conceivablytranspire without ventricular activity. Thus, one option is to not usethis aspect of the mode supervisor protocol with the “feed forward”protocol.

Another option is to utilize portions of the “feed back” protocol withthe “feed forward” protocol in certain circumstances. For example, thefar field sense during the ABP of interval 2 is deemed valid. Because ofthis, the far field sense during the ABP of interval 3 is deemedinvalid. When this condition occurs, the mode supervisor MS may thenrevisit the previous interval to determine if a second ventricular eventoccurred. If not, the validity of the interval 2 ABP sense may bereclassified.

It is to be understood that the above description is intended to beillustrative and, not restrictive. Many other embodiments will beapparent to those of skill in the art upon reading and understanding theabove description. The scope of the invention should, therefore, bedetermined with reference to the appended claims, along with the fullscope of equivalents to which such claims are entitled.

1. An implantable medical device comprising: means for atrial pacing andsensing; means for ventricular pacing and sensing; means for classifyingventricular sensed events from a ventricular channel occurring during ablanking period immediately following an atrial event, wherein theventricular sensed events are classified as conducted events or farfield sensing.
 2. The implantable medical device of claim 1, wherein themeans for ventricular pacing and sensing further include means forimplementing a ventricular pacing minimization protocol wherein acomplete cardiac cycle is permitted to transpire without ventricularactivity while maintaining AV synchrony.